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Correction on geometric sampling

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This commit is contained in:
Yinan Zhang 2020-03-04 10:27:30 -08:00
parent 6c3491ad31
commit 305b1f6d96
1 changed files with 8 additions and 1 deletions
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9
src/prof.c
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@ -468,9 +468,16 @@ prof_sample_threshold_update(tsd_t *tsd) {
* Springer-Verlag, New York, 1986 * Springer-Verlag, New York, 1986
* pp 500 * pp 500
* (http://luc.devroye.org/rnbookindex.html) * (http://luc.devroye.org/rnbookindex.html)
*
* In the actual computation, there's a non-zero probability that our
* pseudo random number generator generates an exact 0, and to avoid
* log(0), we set u to 1.0 in case r is 0. Therefore u effectively is
* uniformly distributed in (0, 1] instead of [0, 1). Further, rather
* than taking the ceiling, we take the floor and then add 1, since
* otherwise bytes_until_sample would be 0 if u is exactly 1.0.
*/ */
uint64_t r = prng_lg_range_u64(tsd_prng_statep_get(tsd), 53); uint64_t r = prng_lg_range_u64(tsd_prng_statep_get(tsd), 53);
double u = (double)r * (1.0/9007199254740992.0L); double u = (r == 0U) ? 1.0 : (double)r * (1.0/9007199254740992.0L);
uint64_t bytes_until_sample = (uint64_t)(log(u) / uint64_t bytes_until_sample = (uint64_t)(log(u) /
log(1.0 - (1.0 / (double)((uint64_t)1U << lg_prof_sample)))) log(1.0 - (1.0 / (double)((uint64_t)1U << lg_prof_sample))))
+ (uint64_t)1U; + (uint64_t)1U;